Conventionally, in radiographic x-ray equipment and x-ray computed tomography (CT) for medical use, a grid to remove scattered radiation (scattered radiation removing means) has been used in order to prevent the scattered x-rays (hereinafter termed “scattered radiation”) from the test subject from being incident on the x-ray detector. However, even when a grid is used, a false image is formed by the scattered radiation that passes through the grid, and a false image is formed by the absorbing foil that forms the grid. In particular, when a flat panel (two-dimensional) x-ray detector (FPD: Flat-Panel Detector) wherein detecting elements are arranged in a grid (a two-dimensional matrix) is used as the x-ray detector, the false images such as moire fringes that are produced due to the differences in the spacing between the absorbing foil in the grid are produced in addition to those false images from scattered radiation. False image correction is necessary to reduce these false images. Additionally, recently there has been a proposal for a synchronized grid wherein the layout direction is parallel to either the row or column direction of the detecting elements and there is absorbing foil that is disposed at integer multiples of the pixel spacing of the FPD, so that such moire fringes will not be produced, and there is the need for also for a correction method used therewith. (See, for example, Japanese Unexamined Patent Application Publication 2002-257939.)
While, at present, in methods that perform image processing that includes smoothing, and the like, for correcting the moire fringes, there is a tendency to have a reduction in the resolution of the direct x-rays (hereinafter termed the “direct radiation”) when there is excessive correction of false images. As a result, the images wherein the direct radiation x-rays have been reduced, wherein the attempt has been made to reduce the false images reliably during the image processing, do not become more clear, and, conversely, when the emphasis is on the resolution of the direct x-rays in an attempt to make the image more clear, then there will be no reduction in the false images during the image processing, in the so-called trade-off between image processing and clarity. Because of this, it is difficult to perform adequate processing on the false images. Furthermore, while there have been a variety of methods proposed as methods for making corrections for the scattered radiation that remains even when a grid is used, there is a problem in that the correction calculations are time-consuming.
The present applicants have already proposed a method for making corrections using a synchronized grid, to correct pixels wherein the direct radiation is blocked by the absorbing foil, to calculate a distribution for the scattering that has been transmitted through the grid from the blocked pixel column or pixel row, to correct the other pixel signals based on that distribution. Additionally in this method there were proposals for, for example, having the distance between the grid and the x-ray detector be an integer multiple of the height of the absorbing foil, radiation emitting means such as an x-ray tube, the establishment of a grid position, and an absorbing foil shaped so as to cause the shadow of the absorbing foil to be contained within a constant pixel column or pixel row.
However:
1. Usually grids other than synchronized grids are used, and when they are used, the method described above, proposed by the present applicants, is inapplicable.
2. No thought has been given to the impact due to misalignment due to a deformation of the absorbing foil from which the grid is structured, even if a synchronized grid is used, or to the misalignment in position or orientation of the grid as a whole that occurs due to the absorbing foil that structures the grid and the rows and columns of the detector not being perfectly parallel.
3. While in the method described above, proposed by the present applicant, the application of the correction is limited to the pixel column or pixel row blocked by the absorbing foil, the direct radiation is attenuated through absorption by the grid structure other than the absorbing foil, for the other pixel columns or pixel rows. Specifically, a grid cover that covers the absorbing foil, and the like is actually formed by carbon fibers or an aluminum plate, so there is absorption by the grid cover. Additionally, when the middle material, disposed between the absorbing foil and through which the x-rays are transmitted, is formed from aluminum or from an organic substance, there is absorption by the intermediate material. Because the absorption and attenuation thereby is not taken into account, it is possible that there will be an error in the evaluation of the estimated direct radiation intensities for all of the pixels, including the blocked pixels.
4. Furthermore, in the method described above, proposed by the present applicant, the distribution of the scattered radiation that is transmitted through the grid is calculated from the blocked pixel columns or pixel rows, and the signals for the other pixels are corrected based on that distribution, but the effect on the scattered radiation by the positional misalignment due to deformation of the absorbing foil is not taken into account. Consequently, there is the danger that, conversely, false images will be produced through error in the scattered radiation intensity distribution due to the effect thereof
5. Moreover, in the method described above, proposed by the present applicant, the distance between the grid and the x-ray detector is set to an integer multiple of the height of the absorbing foil so that the sums of the viewing angles of the grid will be essentially identical from each pixel (where, in this method, an example is given wherein the distance between the grid and the x-ray detector is the same as the height of the absorbing foil). However, in practice there is an angular distribution in the grid viewing angles for the individual pixels, and the actual scattered radiation intensities will have intensity distributions in accordance with the viewing angle. Consequently, there is a high likelihood that there will be error in the scattered radiation intensity distribution calculated in the correction calculations.
6. Additionally, in the method described above, proposed by the present applicant, the grid position and shape of the absorbing foil are set so that the shadow of the absorbing foil will be contained within a constant pixel column or pixel row, even if there is a change in the position of the radiation emitting means, the grid, and the x-ray detector, but actually, often in, for example, a circulatory system imaging device, the distance between the radiation emitting means, the grid, and the x-ray detector are changed each time the imaging is performed. Consequently, use is only possible within a limited range in order to fulfill the conditions so that the shadow of the absorbing foil will be contained with in a constant pixel column or pixel row. Furthermore, it is necessary to be within the requirements for the deformation for the absorbing foil that structures the grid, and for the misalignment from the design position, and thus there are tight requirements for the mechanical precision and assembly precision, which are difficult to achieve from a cost perspective and technology perspective.
The present invention is the result of contemplation on this type of situation, and the object thereof is to provide a radiography device that can be applied also to general-use scattered radiation removal means, and capable of producing appropriate radiation images that are independently of the status of installation of the scattered radiation removing means.